C.M. Bradford 1, G.J. Stacey 1, R.J. Cohen 2, J. Fischer 3, M.A. Greenhouse 4, S. D. Lord 5, D. Lutz 6, R. Maiolino 7, A. Malkan 8, N. Rieu 9, H. A. Smith 10, L. Spinoglio 11, & S. J. Unger 12
1 Cornell University
2 Jodrell Bank
3 Naval Research Laboratory
4 NASA Goddard Space Flight Center
5 California Institute of Technology
6 MPE, Garching
7 Osservatorio Astronomico di Arcetri
8 University of California, Los Angeles
9 Observatoire de Paris
10 Smithsonian Astrophysical Observatory
11 CNR-Istituto di Fisica dello Spazio Interplanetario
12 Queen Mary and Westfield College
We present ISO SWS and LWS observations of OH in the molecular gas associated with the starburst nucleus of NGC 253. Using the LWS and SWS gratings, we have detected absorption in three of the four rotational lines that connect to the ground rotational state: the 119 um, the 53.3 um, and the 34.6 um doublets. There is no emission or absorbtion apparent in the fourth doublet at 79 um. We also observe rotationally excited emission of OH at 163 um. The 119 um doublet was also observed with the LWS Fabry-Perot, resulting in a velocity resolved spectrum.
The absorption line widths indicate a large column of OH
(
)
outside the nucleus,implying an OH abundance
assuming
the molecular column estimated with low J CO line studies. When compared
with the 18 cm radio absorption profile, the OH column density implies an 18 cm
excitation temperature of
,
constraining models for the excitation
of the radio lines, and hence the physical conditions of the molecular gas.
The photon rates in the 53.3 and 34 um absorption lines and the 163 um
emission line indicate that the
:
J = 3/2
level is radiatively populated
by the strong far-IR continuum. The line strengths and upper limits of the
undetected lines constrain the geometry of the molecular material, and its
relationship to the FIR-emitting dust.
We also present ISO LWS observations CH (149 um) and H2O (179 um), and discuss their implications of these detections for models of the molecular ISM in NGC 253.